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Cover Page - shares
 9 Months Ended
Sep. 30, 2021
Oct. 31, 2021
Document Information [Line Items]    
Amendment Flag false  
Document Type 10-Q  
Document Period End Date Sep. 30, 2021  
Document Fiscal Year Focus 2021  
Document Fiscal Period Focus Q3  
Entity Registrant Name ZAI LAB LIMITED  
Entity Central Index Key 0001704292  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status Yes  
Entity Filer Category Large Accelerated Filer  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Interactive Data Current Yes  
Entity File Number 001-38205  
Entity Incorporation, State or Country Code E9  
Entity Address, Address Line One 4560 Jinke Road  
Entity Address, Address Line Two Bldg. 1, Fourth Floor Pudong  
Entity Address, City or Town Shanghai  
Entity Address, Country CN  
Entity Address, Postal Zip Code 201210  
Document Quarterly Report true  
Document Transition Report false  
Entity Tax Identification Number 98-1144595  
Entity Small Business false  
Local Phone Number 6163 2588  
City Area Code 86 21  
American Depositary Shares    
Document Information [Line Items]    
Trading Symbol ZLAB  
Entity Common Stock, Shares Outstanding   68,893,502
Title of 12(b) Security American Depositary Shares, each representing 1 Ordinary Share, par value $0.00006 per share  
Security Exchange Name NASDAQ  
Ordinary Shares    
Document Information [Line Items]    
Trading Symbol 9688  
Entity Common Stock, Shares Outstanding   96,408,743
Title of 12(b) Security Ordinary Shares, par value $0.00006 per share